In a conventional chemical reaction process, there was a demand of improvement in efficiency by scale-up. In recent years, however, a microreactor, which is a reactor having been scaled down, receives attention owing to such demands as reduction of environmental burdens, resource saving and energy saving. In particular, it becomes a much more real possibility associated with progress of microfabrication technique and microanalysis technique.
It has been known that the following advantages are obtained by reducing a reactor in size. (1) A microreactor has a small reaction space owing to the small dimension thereof. Accordingly, the diffusion distance of molecules is short to facilitate quick molecule migration on mixing, extraction and the like, whereby a period of time required for reaction and extraction can be shortened. (2) Reaction and molecule migration at an interface between liquids or between liquid and solid undergoes efficiently owing to a larger surface area per unit volume. (3) Heat exchange is quickly conducted owing to a small heat capacity of a liquid in the channel, whereby a uniform reaction temperature can be easily maintained, and quick heating and cooling can be easily conducted.
As having been described, a microreactor has such characteristics that are not found in a large reaction device, and is being expected to exert great contribution on synthesis reaction experiments for screening chemical reagents and the like. It is also expected to exert contribution on development of a novel chemical process. In a part of the fields of fine chemicals, it is also expected to produce a product industrially by using a microreactor.
However, associated with reduction in size of the reaction field, such a flow phenomenon is exhibited that is different from those in a conventional macroscopic reaction. For example, in the case where the diameter of the channel is considerably reduced, the Reynolds number is decreased to make a laminar flow dominant. Accordingly, such a flow phenomenon is exhibited that is largely different from those in a turbulent flow caused by mechanical agitation in a macroscopic scale. Furthermore, the surface tension has large influences since the surface area per volume is increased. Consequently, a flow state that is largely different from those in a macroscopic reaction is exhibited, and it therefore is important to control the state certainly.
Various proposals have been made in a microreactor on measures for accelerating mixing of a reaction liquid. For example, Patent Document 1 discloses a method of mixing a plurality of reactants, which are divided into a number of channels respectively, by feeding them to a reaction chamber. Patent Document 2 discloses a method of mixing by agitation with an ultrasonic vibrator. Patent Document 3 discloses that a minute agitator formed of carbon nanotubes is rotated in a concave part of a channel to attain agitation, whereby a laminar flow is changed to a turbulent flow to shorten the reaction time. In the case where these methods are employed, however, it is necessary to make the configuration of the channel complex, or to dispose a vibrator or an agitator for mixing, whereby the device are not necessarily designed easily. Furthermore, in the case where liquids that are incompatible with each other are dispersed too finely, there is such a possibility that it is difficult to separate the liquids again for recovery.
After reacting two kinds of liquids incompatible with each other by mixing in a microreactor, it is preferred that the liquids are again separated and recovered after the reaction in consideration of reuse of the solvent and purification and analysis of a product. Patent Document 4 discloses a method of merging two kinds of reaction liquids introduced from two introduction ports, then conducting an interface reaction while the liquids are in contact with each other in a laminar flow state, and then separating the liquids for recovering from two recovering ports. In this method, the liquids are handled as a two-phase flow, but it is considered that it is not necessarily easy to separate and recover them with good reproducibility. Non-patent Document 1 discloses a method of merging two liquids different in specific gravity, and then separating the two liquids in a settler (pool) for recovery. However, the flow state in the channel greatly influences recovering capability. The flow state is liable to change due to the performance of the pump and flow rate, and thus it is not easy to handle the liquids with good reproducibility.
Non-Patent Documents 2 and 3 disclose a method for reacting two kinds of liquids incompatible with each other by flowing them alternately in a channel. According to the method, an alternating flow can be formed with good reproducibility in a state where a laminar flow is dominant, and the specific interface area between the two kinds of liquids can be relatively large. However, it is necessary to use a special pump capable of feeding minute amounts of liquids alternately, which impairs size reduction and simplification of the device.    Patent Document 1            JP 9-512742 T            Patent Document 2            JP 11-347392 A            Patent Document 3            JP 2004-321063 A            Patent Document 4            JP 2004-181298 ANon-Patent Document 1        
Akinori Muto, other 3 persons, “Y-Pattern Microchannel Reactor with Settler and Extraction Properties of Copper Ion”, Kagaku Kogaku Ronbunshu, vol. 30, No. 2, pp. 159-163 (March of 2004)
Non-Patent Document 2
Hideho Okamoto, “Micro Hanno Kagaku ni kansuru Jitsuyoka no Doko (Trends on Practical Application of Microreaction Chemistry)”, KagakuSochi, vol. 46, No. 9, pp. 74-80 (September of 2004), published by Kogyo Chosakai Publishing, Inc.
Non-Patent Document 3
Hideho Okamoto, other 2 persons, “New method for increasing productivity by using microreactors of planar pumping and alternating pumping types”, Chemical Engineering Journal, vol. 101, No. 1-3, pp. 57-63 (August of 2004)